HPV is a small del DNA virus having an icosahedral structure and no envelope. The genome of the virus contains open reading frames (ORFs) called E1-E7 and L1 and L2: “E” means early, and “L” means late. L1 and L2 encode capsid proteins of the virus. The early (E) genes are associated with functions such as virus replication and cell transformation. The L1 protein is the major capsid protein having a molecular weight of from 55 to 60 kDa when measured by polyacrylamide gel electrophoresis. The L2 protein is the minor capsid protein which also has an estimated molecular weight of from 55 to 60 kDa and an apparent molecular weight of from 75 to 100 kDa.
Although the mortality from cervical cancer has recently decreased in developed countries, it is the fifth leading cause of malignancy deaths and the second most common malignancy in women worldwide. Certain sexually transmitted types of HPV are the most important risk factor for cervical cancer. Recent reports show that from 30 to 50% of young women who recently had their first sexual intercourse have HPV infection in the cervix. Surprisingly, most cervical HPV infections are caused by high-risk types of HPV which can induce cancer. HPV types 16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 67 and 68, possibly some other types as well, are considered to be high-risk types.
This alarmingly high prevalence of HPV infection among young women suggests that educational and social health programs aimed at preventing of HPV infection may not be sufficiently effective in combating cervical cancer. Especially, prevention of infection with high-risk types of HPV is a priority for women in developing countries and young women who have not had uterine cervical cancer screening. The current cytologic screening (uterine cervical cancer screening) and post-onset cancer treatment are not cost-effective choices. Nationwide use of prophylactic vaccines against high-risk types of HPV can decrease the incidence of cervical cancer. It is estimated that even single use of a HPV16 vaccine decreases cervical cancer by half.
In pursuit of development of HPV vaccines, it was reported that high-level production of the HPV11 L1 protein led to the assembly of virus-like particles (hereinafter sometimes referred to simply as “VLPs”) in an insect cell system (non-patent document 1). Successful synthesis of HPV16 VLPs in this insect cell system was also reported (non-patent document 2).
Later, we succeeded in production of HPV6- and HPV16-derived VLPs in the fission yeast Schizosaccharomyces pombe (hereinafter sometimes referred to simply as “S. pombe”) (non-patent document 3). Although the yield of VLPs from the fission yeast is less than that from the insect cell system, the expression system using the fission yeast confers advantages in large-scale production of virus-like particles (hereinafter sometimes referred to simply as “VLP”) and safety of use in humans.
Koustsky et al. were the first to report that parenteral vaccination (by injection) with HPV16 VLPs conferred 100% protection against HPV16 infection in is women (non-patent document 4).
Unfortunately, the injectable HPV16-VLP vaccine is expensive because it requires advanced techniques and special facilities for its production and storage. In addition, repetitive injection vaccinations required for efficacy has limited feasibility and are impractical in developing countries with limited numbers of trained members in their clinical staffs. Furthermore, it has been reported that injection vaccination with VLPs is a poor inducer of secretory IgA, which plays a major role in mucosal immunity (non-patent document 5). Immunization of the mucosa-associated lymphoid tissue (MALT), which is an immune tissue located in the respiratory and the digestive tract, can protect against viruses such as HPV that cause infections in the uterine and the vaginal mucosal epithelia. Balmelli et al. succeeded in inducing mucosal antibodies that neutralize HPV16 in the vagina by intranasal administration of HPV16-VLPs (non-patent document 6).
However, intranasal vaccination is also problematic like injection vaccination, because it requires preparation of relatively large amounts of purified HPV-VLPs. Stimulation of the gut-associated lymphoid tissue (GALT) with edible human papilloma virus vaccines (hereinafter sometimes referred to simply as “edible HPV vaccines”) was attempted to induce strong mucosal immunity in the vagina. Two groups have produced edible HPV vaccines from tobacco and potato plants that express the HPV11 (non-patent document 7) and HPV16 (non-patent document 8) L1 genes.
Purification of HPV virus-like particles (HPV-VLPs) was disclosed in patent document 1. Patent documents 2-4 and patent document 5 disclose HPV vaccine preparations from expression systems in baculovirus and in insect cells, respectively. A nucleic acid vaccine for immunotherapy of HPV was also disclosed (patent document 6).
A therapeutic microorganism delivery system using non-vaccine active ingredient was also reported (patent document 7).    Patent Document 1: JP-A-2003-520188    Patent Document 2: JP-A-2001-519161    Patent Document 3: JP-A-2002-516291    Patent Document 4: JP-A-2002-510976    Patent Document 5: JP-A-2004-269    Patent Document 6: JP-A-2004-121263    Patent Document 7: JP-A-10-506791    Non-patent Document 1: Rose R C, et al. J Virol. 1993; 67: 1936-44.    Non-patent Document 2: Kirnbauer R, et al. J Virol. 1993; 67: 6929-36.9    Non-patent Document 3: Sasagawa T, et al. Virology. 1995; 206:126-35.    Non-patent Document 4: Koutsky L A, et al. N Engl J Med. 2002; 347: 1645-51.    Non-patent Document 5: Hagensee M E, et al. Virology 1995; 206: 174-82.    Non-patent Document 6: Balmelli C, et al. J Virol 1998; 72: 8220-9.    Non-patent Document 7: Warzecha H, et al. J Virol. 2003; 77: 8702-11.    Non-patent Document 8: Biemelt S, et al. J Virol. 2003; 77: 9211-20.